Silicon tetrafluoride is a by-product recoverable from the phosphate fertilizer industry and has numerous applications which make it a commercial commodity. In the normal recovery of silicon tetrafluoride from fluosilicic acid, the silicon tetrafluoride becomes contaminated with significant amounts of hydrogen halides such as hydrogen chloride or hydrogen fluoride which form during the beneficiation from phosphate ores.
Silicon tetrafluoride is a difficult chemical to handle in that it is acidic and has a very low boiling point, thus limiting the use of materials for contact with it. There exists a need to provide materials which may contact the silicon tetrafluoride without significant degradation caused by the acid character of the silicon tetrafluoride. There also exists a need to remove the hydrogen halide contamination present in the silicon tetrafluoride. The presence of hydrogen halides is detrimental in certain processes where silicon tetrafluoride is a reactant.
Similarly, it has been found that silicon tetrafluoride sometimes contains significant amounts of sulfur dioxide (SO.sub.2) contaminant which has odor problems and can cause problems in subsequent processing of the silicon tetrafluoride. Thus there exists a need for a material which may be contacted with silicon tetrafluoride and will remove sulfur dioxide.
Certain naturally occurring hydrated metal aluminum silicates are called zeolites. Some of these are called mordenites. The synthetic adsorbents of the invention have compositions similar to some of the natural mordenites. The most common of the zeolites are sodium zeolites. Zeolites are useful as adsorbents, detergent builders, cracking catalysts and molecular sieves. Mordenites of the invention are particularly useful as adsorbents.
Zeolites consist basically of a three-dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra. The tetrahedra are crossatoms linked by the sharing of oxygen atoms so that the ratio of oxygen atoms to the total of the aluminum and silicon atoms is equal to two or oxygen/(Al+Si)=2. The electrovalence of each tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example, a sodium ion. This balance may be expressed by the formula Al/Na=1. The spaces between the tetrahedra are occupied by water molecules prior to dehydration.
Mordenite may be distinguished from other zeolites and silicates on the basis of X-ray powder diffraction patterns and certain physical characteristics. The X-ray patterns for mordenite are described hereinafter. Composition and density are among the characteristics which have been found to be important in identifying zeolites.
The basic formula for all crystalline sodium zeolites may be represented as follows: EQU Na.sub.2 O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
In general, a particular crystalline zeolite will have values for "x" and "y" that fall in a definite range. The value "x" for a particular zeolite will vary somewhat since the aluminum atoms and the silicon atoms occupy essentially equivalent positions in the lattice. Minor variations in the relative numbers of these atoms do not significantly alter the crystal structure of physical properties of the zeolite. For mordenite an average value for "x" is about 10.0 with the "x" value usually falling within the range 10.0.+-.1.00 However, mordenites may be modified to values "x" far in excess of these limits.
The value of "y" is not necessarily an invariant for all samples of zeolites. This is true because various exchangeable ions are of different size, and since there is no major change in the crystal lattice dimensions upon ion exchange, the space available in the pores of the zeolite to accommodate water molecules varies.
The formula for mordenite may be written as follows: EQU 0.9.+-.0.2Na.sub.2 O.Al.sub.2 O.sub.3.10.0.+-.1.5 SiO.sub.2.yH.sub.2 O
and wherein "y" may be any value up to 9.
The pores of zeolites normally contain water.
The above formula represents the chemical analysis of mordenite. When other materials as well as water are in the pores, chemical analysis will show a lower value of "y" and the presence of other adsorbates. The presence in the crystal lattice of materials volatile at temperatures below about 600.degree. C. does not significantly alter the usefulness of the zeolites as an adsorbent since the pores are usually freed of such volatile materials during activation.
Among the ways of identifying zeolites and distinguishing them from other zeolites and other crystalline substances, the X-ray powder diffraction pattern has been found to be a useful tool. In obtaining X-ray powder diffraction patterns, standard techniques are employed. The radiation is the K.alpha. doublet of copper, and a Geiger counter or proportional counter spectrometer with a strip chart pen recorder is normally used. The peak heights, I, and the positions as a functions of 2.theta. where .theta. is the Bragg angle, are read from a spectrometer chart. From these, the relative intensities, 100I/I.sub.O, where I.sub.O is the intensity of the strongest line or peak, and "d" the interplanar spacing in Angstroms (.ANG.) corresponding to the recorded lines are calculated.
X-ray powder diffraction data for sodium mordenite are given in Table A. Relative intensity, 100I/I.sub.O and the "d" values in Angstroms (.ANG.) for the observed lines are shown. In a separate column are listed the Miller indices (h, k, l) for an orthohombic unit cell that corresponds to the observed lines in the X-ray diffraction patterns.
TABLE A ______________________________________ SYNTHETIC LARGE-PORE NA-MORDENITE (A) 100I/I.sub.o h,k,l ______________________________________ 13.4 40 110 10.2 10 020 9.02 70 200 6.50 50 111 6.32 30 130 6.02 10 021 5.75 20 201 5.03 2 221 4.84 2 131 4.50 35 330 4.12 5 041 3.97 70 420 3.81 15 150 3.73 10 241 3.52 10 002 3.45 100 112 3.37 60 510 3.28 10 022 3.21 55 202 3.13 10 060 ______________________________________
The particular X-ray technique and/or apparatus employed, the humidity, the temperature, the orientation of the powder crystals and other variables, all of which are well known and understood to those skilled in the art of X-ray crystallography or diffraction can cause some variations in the intensities and positions of the lines. These changes, even in those few instances where they become large, pose no problem to the skilled X-ray crystallographer in establishing identities. Thus, the X-ray data given herein to identify the lattice for mordenite are not to exclude those materials which, due to some variable mentioned or otherwise known to those skilled in the art, fail to show all of the lines, or show a few extra ones that are permissible in the orthorhombic system of that zeolite, or show a slight shift in position of the lines, so as to give a slightly larger or smaller lattice parameter.
The sodium form of mordenite may be treated with ammonium cation (NH.sub.4.sup.+) to replace some of the sodium cation. Treatment with ammonium is followed by heating to drive off the gaseous ammonia (NH.sub.3). Quite typically, ammonium nitrate, NH.sub.4 NO.sub.3 or ammonium chloride, NH.sub.4 Cl or similar materials are used as the source of ammonium cation to replace the sodium cation in the mordenite. Alternatively, mordenite may be treated with a dilute strong acid to leach out some of the sodium and substituting a proton. Although the hydrogen cation form has an effect on the X-ray diffraction pattern and other characteristics of the mordenite (in comparison to the sodium form), the changes are minor and well-recognized by one skilled in the art. An example of a commercially available mordenite of the hydrogen cation form is Zeolon.RTM. 900H. This product and those like it are known to be thermally stable and acid resistive or acid stable.
A simple test described in "American Mineralogist," Vol. 28, page 545, 1943, permits a quick check of the silicon to aluminum ratio of the zeolite. According to the description of the test, zeolite materials with a three-dimensional network that contains aluminum and silicon atoms in an atomic ratio of Al/Si=2/3 =0.67, or greater, produce a gel when treated with hydrochloric acid. Zeolites having smaller aluminum to silicon ratios disintegrate in the presence of hydrochloric acid and precipitate silica. These tests were developed with natural zeolites and may vary slightly when applied to synthetic types.